Coating for magnesium and its alloys and method of applying

ABSTRACT

The surface of magnesium and its alloys is provided with a protective coating, both decorative and resistant to corrosion, when subjected to an ammonium bifluoride and an ammonium metavanadate aqueous solution which preferably contains in addition water-soluble organic compounds, water-soluble permanganate ions, or water-soluble nitrate ions. The coating is conveniently and efficiently applied to the metal which is the anode in the coating solution.

O United States Patent 1 113,620,939

I 72] Inventor Ocke C. Fruchtnicht FOREIGN PATENTS Huntsville All528.234 10/1940 Great 1311mm 204/56 pp No 807.923 387.437 2 1933 GreatBritain 204/56 [22] Filed Mar. 17, I969 [45] Patented No 16, 1971Primary Examu er.lohn H. Mack [73] Assignee The United States of Americaas Amsmn' f mmmer-R- Anfirews represented by the Secretary 0 the ArmyAnorneys Harry M. Saragovnz, Edward Jr Kelly, Herbert Berl and James T.Deaton [54] COATING FOR MAGNESIUM AND ITS ALLOYS AND METHOD OF APPLYING12 Claims, No Drawings ABSTRACT: The surface of magneslum and its alloys1s pro- [52] U.S. Cl .1 204/56 M vided with a protective coating hdecorative d i m C23! 9/06 to corrosion, when subjected to an ammoniumbifluoride and of Search an ammonium metayanadate aqueous olutionpreferably contains in addition water-soluble organic com- [56]References Cited pounds, water-soluble permanganate ions, orwater-soluble UNITED STATES PATENTS nitrate ions. The coating isconveniently and efficiently ap- 2,313,755 3/1943 Loose v. l48/6 pliedto the metal which is the anode in the coating solutionv COATING FORMAGNESIUM AND ITS ALLOYS AND METHOD OF APPLYING BACKGROUND OF THEINVENTION Magnesium metal and its alloys are attractive for structuralmaterials; however, because of its reactivity and tendency todeteriorate by corrosion, the metal and its alloys most generally mustbe provided with a protective coating. Existing TABLE I.CHEMICALCOMPOSITIONS OF MAGNESIUM ALLOYS [Alloy content, weight percent] Al ZnMn Ni 1 Cu Fe I 1 Max. 3 Min.

coatings have provided a certain degree of protection by delaying theonset of corrosion. The best state'of-the-art coatings offer noprotection at points where the substrate metal is exposed by penetrationsuch as by scratching, scribing through the coating, incomplete coveringor failure of the coating to provide a physical barrier to theenvironment. Imperfections in the coatings such as those which may bepresent after the most careful application procedure of the prior art orthose resulting from mechanical abuse after application, serve as afocal point for the onset of corrosion. Once begun, this corrosionaccelerates rapidly due to the lack of inhibitive properties in thecoating other than a barrier to the atmospheric conditions by itsphysical presence. Thus, a need exists for a coating which, in additionto providing a physical barrier to the environment, will provide aninhibiting influence to counteract corrosion both at breaks in thecoating and at porosity sites where moisture or other constituentspresent in the atmosphere eventually penetrate through to the substratemetal.

Therefore, an object of this invention is to provide a coating which isattractive and which provides a high degree of protection from corrosionto a substrate of magnesium and its alloys.

Another object of this invention is to provide a protective coating formagnesium and its alloys which coating will extend its protectiveinfluence over minor points of penetration through the coating.

A further object of this invention is to provide a method for producingan anodic reaction on the surface of magnesium and its alloys to therebyproduce thereon a hard brown to gray colored uniform coating of highprotective value.

A still further object of this invention is to provide a novel aqueoussolution of chemicals for use in coating magnesium and its alloys.

SUMMARY OF THE INVENTION This invention relates to a method forelectrochemically applying a protective coating to magnesium and itsalloys. A decorative and corrosion-resistant coating is formed onmagnesium and its alloys by an anodic reaction in a solution or aqueousbath containing ammonium bifluoride, ammonium meta-vanadate, andpreferably containing a third ingredient selected from polar organiccompounds, sodium or potassium permanganate, and sodium or potassiumnitrate. The solution is used in conjunction with an impressed potentialof approximately 135-1 75 DC volts or 140-[90 AC volts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The coating process of thisinvention works best with magnesium alloys which do not contain a largeamount of lithium (e.g. l3-l5 weight percent, such as alloy LA I 4 l XAof table II below).

Magnesium alloys are those alloys which contain a major portion ofmagnesium, generally those containing at least TABLE II.AVAILABLE FORMSOF MAGNESIUM ALLOYS Sheet and plate Castings Forg- Welding lngs rod Allforms of magnesium alloys may be employed in the process of thisinvention; however, those of high lithium content do not coat as well asalloys having little or no lithium. Over 30 magnesium alloys areavailable as castings and/or wrought products including sheet, plate,extrusions, and forgings. Alloys which have been processed with varioushardening and annealing characteristics are available. An example of analloy designation might be AZ3lB-H24, indicating an alloy containing 3percent aluminum, 1 percent zinc which has been strain hardened, andpartially annealed. For additional information on magnesium alloys,refer to Defense Metals Information Center, Report 206, dated Aug. 26,I964.

The representative composition of the aqueous bath for the coatingprocess is as follows:

Ammonium bifluoride in range of 32 oz. to 52 oz. per gallon; 42oz./gallon being preferred;

Ammonium mcta-vanadate in range of 8 oz. to 14 oz. per gallon; l loz./gallon being preferred;

Water-soluble organic, permanganate. or nitrate ions- 2.530 oz./gallon.

The water-soluble organic compounds may be selected from polar organiccompounds such as ethanol, sodium potassium tartrate, citric acid,ethylene glycol, dimethylsulfoxide, N,N-dimethylformamide,dimethylenetriamine, ethanolamine, allyl alcohol, and piperidine.Potassium or sodium permanganate works well for the source of thepermanganate ion. Also, potassium or sodium nitrate is suitable for asource of the nitrate ion.

The solution for the coating process may be comprise of only thebifluoride and the meta-vanadate; however, the coating will form onlyslowly and is not as protective as coatings containing one of the thirdconstituents (e.g., water-soluble polar compounds, organic, permanganateions or nitrate ions).

The impressed potential of approximately 135-175 DC volts or 140-190 ACvolts required in the process of this invention is preferably increasedfrom zero gradually to the desired value in order to allow time for thehigh dielectric coating to form which has a tendency to depress thecurrent flow. A rheostat is used to regulate the current flow and tomaintain current density. A current density of 24 a./sq. ft. of theanode surface emersed in the solution works well for the process of thisinvention. A wide range of current densities, however, may be used.Higher current densities will coat faster, but heating may becomeexcessive. Maintaining ambient temperatures for the solution isdesirable, although some heating is not harmful. The temperature of thesolution must not, however, be allowed to approach boiling as somedecomposition will take place. The metal is subjected to the anodicreaction, and the treatment is carried out until the current flow dropsto or near zero value at the given potential. When employing a currentdensity of 24 a./sq. ft. and cooling the solution, coatings areaccomplished in approximately 15 to 20 minutes.

The following descriptions and examples illustrate the use of thisinvention and sets forth the preferred embodiments. Changes ormodifications within the scope of this invention may be found desirableby those skilled in its application.

EXAMPLE I A group of AZ-3lB magnesium alloy coupons or metal piecesapproximately one-half inch by 4 inches by 0.065 inch are vapordegreased in trichloroethylene and then acid etched in a bathconstituted of 8 fluid ounces of nitric acid, 2 fluid ounces of sulfuricacid, and 90 fluid ounces of distilled water. After rinsing, one of thecoupons is made the anode, and another coupon is made the cathode of aDC power source which is capable of at least 175 volts and amperesoutput. The coupons are then immersed or contacted with an aqueous bathor a solution of the following composition:

Ammonium bifluoride 42 oz./gal.

Ammonium meta-vanadate l l oz./gal.

Ethanol 32 fluid ounces/gallon The potential is gradually increased bythe use of the rheostat maintaining a current density of approximately24 a./sq. ft. until a potential of 15 volts is reached, requiringapproximately 10 minutes. At 150 point, the system is allowed to remainfor an additional 10 minutes, during which time the current densitydrops to or near zero. The anode is then removed and insed in water. Theresultant coating is dark brown, hard, smooth, and not easily damaged.

EXAMPLE ll Magnesium alloy coupons are prepared and cleaned as inexample I. The basic solution of ammonium bifluoride 42 ounces/gallonand of ammonium meta-vanadate l l ounces/gallon is prepared and dividedinto a number of smaller solutions of 200 milliliters each. Into each200 milliliters portion is added one of the constituents listed in tableII] (below) to complete the aqueous bath for use in the experiments ofthis example. Coupons were treated in the same manner as example I using135-175 DC volts.

AC potential works quite well, and AC potential may be substituted forDC potential. However, the DC potential appears to give slightly moreconsistent and uniform coatings.

TABLE Ill Appearance of Amount Compound Coating None Gray 4 g. PotasliumPermanganate Khaki Tan 4 g. Sodium Potassium Tartrate Dark Brown 4 g.Citric Acid Brown 20 ml. Ethylene Glycol Dark Brown 20 ml.Dimethylsull'oxide Brown 20 ml. N,N-Dimethylformamide Dark Brown I!)rlll. Diethylenetriamine Brown l0 mil. Ethanolamine Brown 30 ml. AllylAlcohol Dark Brown-Gray 20 ml Piperidine Brown The above list of organiccompounds does not exhaust the possibilities which may be employed inthe solutions of this invention, but is merely representative of some ofthe preferred compounds. A polar organic compound of the types listedprovides sufficient conductivity and catalytic action to allow thecoating process to proceed at the desired rate to yield a satisfactoryfinished product.

In order to evaluate the protective efficiency of the coatings, thecoated magnesium alloy coupons were placed in a 5 percent salt spraychamber maintained at 97 F. A diagonal scribe mark through the coatingexposing the substrate metal was placed on each coupon. An untreatedcoupon was included for comparison. After only a few hours the untreatedcoupon was covered with corrosion spots, whereas the treated couponswere unaffected. After 100 hours exposure, the untreated panel was badlycorroded, whereas the treated panels showed only a slight bleaching ofcolor in the coating and no corrosion evidence in the scribe mark. After200 hours, some of the coupons displayed occasional spots ofdiscoloration, although the scribe marks were still free of corrosion.After 300 hours of exposure, some of the spots of discolorationdeveloped very small white protrusions. Even after 400 and 500 hours ofexposure, the increase in corrosion was barely perceptible, much unlikeany other protective coated magnesium where, once begun, corrosion runsrampant.

best understood by a review of the characteristics and properties of thecoating. The coatings of this invention may be characterized anddescribed by its following properties:

a. Unaffected by temperatures above the melting point of magnesiumalloys.

b. A uniform dark brown or gray color with a surface roughnesscomparable to 20-40 microinches R.M.S. (root mean square).

c. Completely nonchalking, with hardness above that penetrable byfingernail.

d. A high resistance to corrosion coating which extends its inhibitinginfluence over minor imperfections and voids in the coating. The exactchemical mechanism responsible for this is not clearly understood, butit is apparently caused by the very slow leaching of a slightly solubleinhibiting ion (vanadate) from the coating.

e. Inhibition properties which significantly reduce the velocity ofattack when corrosion does begin.

lclaim:

1. Method of providing a surface of magnesium and its alloys with adecorative and protective coating, said method comprising subjectingsaid surface of magnesium and its alloys to an aqueous bath of ammoniumbifluoride and ammonium meta-vanadate, impressing a potential of apredetermined value of volts and a predetermined value of currentdensity for a predetermined period of time for anodic reaction to occurand form on said surface a decorative and protective coating.

2. Method of claim 1 and wherein said surface prior to being subjectedto anodic reactions is subjected to vapor degreasing and cleaning; saidaqueous bath comprises ammonium bifluoride about 42 ounces/gallon,ammonium mcta-vanadate about l l ounces/gallon, and an additionalingredient selected The benefits directly attainable from the inventionwill be from the polar compounds consisting of ethanol, sodium potassiumtartrate, citric acid, ethylene glycol, dimethylsulfoxide,N,N-dimethylformamide, dimcthylenetriamine, ethanolamine, allyl alcohol,piperidine, potassium permanganate, sodium permanganate, sodium nitrate,and potassium nitrate in an amount from about 2.5 to about 30ounces/gallon.

3. Method of claim 2 and wherein said surface is vapor degreased intrichloroethylene and said cleaning is done by acid etching in a bathconstituted of about 8 fluid ounces of nitric acid, 2 fluid ounces ofsulfuric acid, and 90 ounces of distilled water; said impressedpotential of a predetermined value of volts is in the range ofapproximately 135-190 volts; said predetennined value of current densityis about 24 a./square foot; said predetermined time is from about toabout minutes; and said protective coating formed is corrosionresistant.

4. Method of claim 3 and wherein said impressed potential of apredetermined value of volts is in the range of approximately 140-190 ACvolts.

5. Method of claim 4 and wherein said impressed potential of about l50AC volts is reached in about 10 minutes and said volts are allowed toremain for about an additional 10 minutes during which time said currentdensity drops to about zero value.

6. Method of claim 3 and wherein said impressed potential of about 150DC volts is reached in about 10 minutes and said volts are allowed toremain for about an additional 10 minutes during which time the saidcurrent density drops to about zero value.

7. An article of magnesium and its alloys covered with a decorative andprotective coating as set forth in the method of claim 1.

8. An article of magnesium and its alloys covered with a decorative andprotective coating as set forth in the method of claim 2 and whereinsaid additional ingredient is ethanol in an amount of 32 fluidounces/gallon.

9. An article of magnesium and its alloys covered with a decorative andprotective coating as set forth in the method of claim 2 and whereinsaid additional ingredient is selected from potassium permanganate,sodium potassium tartrate, and citric acid in an amount of about 4 gramsper 200 milliliters of said aqueous bath.

[0. An article of magnesium and its alloys covered with a decorative andprotective coating as set forth in the method of claim 2 and whereinsaid additional ingredient is selected from ethylene glycol,dimethylsulfoxide, N,N-dimethylformamide, diethylenetriamineethanolamine, allyl alcohol and piperidine in an amount from about l0milliliters to about 20 milliliters per 200 milliliters of said aqueousbath.

1]. An aqueous solution for anodizing magnesium and its alloyscomprising ammonium bifluoride in an amount in the range of 32 oz. to 52oz. per gallon and ammonium metavanadate in an amount in the range of 8oz. to 14 oz. per gallon.

12. The aqueous solution of claim 11 and wherein said ammoniumbifluoride is present in an amount of 42 oz. per gallon, ammoniummeta-vanadate is in an amount of ll oz. per gallon, and an additionalingredient is present in an amount from about 2.5 oz. to about 30oz./gallon; said additional ingredient being selected from the polarcompounds consisting of ethanol, sodium potassium tartrate, citric acid,ethylene glycol, dimethylsulfoxide, N,N-dimethylformamide,dimcthylenetriamine, ethanolamine, allyl alcohol, piperidine, potassiumpermanganate, sodium permanganate, sodium nitrate, and potassiumnitrate.

2. Method of claim 1 and wherein said surface prior to being subjectedto anodic reactions is subjected to vapor degreasing and cleaning; saidaqueous bath comprises ammonium bifluoride about 42 ounces/gallon,ammonium metavanadate about 11 ounces/gallon, and an additionalingredient selected from the polar compounds consisting of ethanol,sodium potassium tartrate, citric acid, ethylene glycol,dimethylsulfoxide, N,N-dimethylformamide, dimethylenetriamine,ethanolamine, allyl alcohol, piperidine, potassium permanganate, sodiumpermanganate, sodium nitrate, and potassium nitrate in an amount fromabout 2.5 to about 30 ounces/gallon.
 3. Method of claim 2 and whereinsaid surface is vapor degreased in trichloroethylene and said cleaningis done by acid etching in a bath constituted of about 8 fluid ounces ofnitric acid, 2 fluid ounces of sulfuric acid, and 90 ounces of distilledwater; said impressed potential of a predetermined value of volts is inthe range of approximately 135-190 volts; said predetermined value ofcurrent density is about 24 a./square foot; said predetermined time isfrom about 10 to about 20 minutes; and said protective coating formed iscorrosion resistant.
 4. Method of claim 3 and wherein said impressedpotential of a predetermined value of volts is in the range ofapproximately 140-190 AC volts.
 5. Method of claim 4 and wherein saidimpressed potential of about 150 AC volts is reached in about 10 minutesand said volts are allowed to remain for about an additional 10 minutesduring which time said current density drops to about zero value. 6.Method of claim 3 and wherein said impressed potential of about 150 DCvolts is reached in about 10 minutes and said volts are allowed toremain for about an additional 10 minutes during which time the saidcurrent density drops to about zero value.
 7. An article of magnesiumand its alloys covered with a decorative and protective coating as setforth in the method of claim
 1. 8. An article of magnesium and itsalloys covered with a decorative and protective coaTing as set forth inthe method of claim 2 and wherein said additional ingredient is ethanolin an amount of 32 fluid ounces/gallon.
 9. An article of magnesium andits alloys covered with a decorative and protective coating as set forthin the method of claim 2 and wherein said additional ingredient isselected from potassium permanganate, sodium potassium tartrate, andcitric acid in an amount of about 4 grams per 200 milliliters of saidaqueous bath.
 10. An article of magnesium and its alloys covered with adecorative and protective coating as set forth in the method of claim 2and wherein said additional ingredient is selected from ethylene glycol,dimethylsulfoxide, N,N-dimethylformamide, diethylenetriamineethanolamine, allyl alcohol and piperidine in an amount from about 10milliliters to about 20 milliliters per 200 milliliters of said aqueousbath.
 11. An aqueous solution for anodizing magnesium and its alloyscomprising ammonium bifluoride in an amount in the range of 32 oz. to 52oz. per gallon and ammonium meta-vanadate in an amount in the range of 8oz. to 14 oz. per gallon.
 12. The aqueous solution of claim 11 andwherein said ammonium bifluoride is present in an amount of 42 oz. pergallon, ammonium meta-vanadate is in an amount of 11 oz. per gallon, andan additional ingredient is present in an amount from about 2.5 oz. toabout 30 oz./gallon; said additional ingredient being selected from thepolar compounds consisting of ethanol, sodium potassium tartrate, citricacid, ethylene glycol, dimethylsulfoxide, N,N-dimethylformamide,dimethylenetriamine, ethanolamine, allyl alcohol, piperidine, potassiumpermanganate, sodium permanganate, sodium nitrate, and potassiumnitrate.